PROJECT SUMMARY/ABSTRACT Recent evidence suggests 5-hydroxytryptamine (5-HT) (also known as `serotonin') has separate central and peripheral skeletal effects. These observations have added to concerns regarding the skeletal effects of widely-prescribed selective serotonin reuptake inhibitors (SSRIs). SSRIs antagonize the 5-HT transporter (SERT) and we have previously shown that genetic and pharmacological disruption of the SERT results in a consistent skeletal phenotype of reduced mass, altered structure and inferior mechanical properties. Similarly, an ever increasing number of clinical studies have reported potential negative skeletal effects of SSRIs, with SSRIs users having less bone mineral density, increased bone loss, and higher rates of fractures than non- users. While there is increasing evidence for a skeletal effect of SERT inhibition, it is currently not known where the effect originates and whether it involves 5-HT. Localizing the skeletal effect of the SERT is important as it may allow the identification of a potentially modifiable pathway that may be targeted to combat off-target skeletal consequences of SSRIs. The overarching hypotheses of the proposed work are that the negative skeletal effects of SERT inactivation are mediated by: 1) centrally-synthesized 5-HT and 2) the sympathetic nervous system via ?-adrenergic receptors. The later finding would indicate that ?-adrenergic receptor antagonists (?-blockers) may be a viable option of reducing the skeletal consequences of SSRIs. To address our overall hypotheses, we will study mice carrying single and double null mutations in the genes encoding the SERT, and tryptophan hydroxylase (Tph)-1 or Tph-2 (Tph1-/- and Tph2-/-). Tph-1 and -2 are responsible for peripheral and central 5-HT synthesis, respectively. In addition, we will selectively inactivate Sert in the central nervous system by crossing transgenic mouse carrying floxed Sert alleles (Sertflox/flox) with mice selectively expressing Cre-recombinase in Tph2-expressing cells (Tph2cre/+). Finally, we will explore whether mice carrying double null mutation in the genes encoding the SERT (Sert-/-) and ?-adrenergic receptor 2 (Adrb2) lack a skeletal phenotype, indicating involvement of the sympathetic nervous system in mediating negative skeletal effects of SERT inactivation. In doing so, we will expose professional graduate students, who are not otherwise exposed to research in their degree program, to meritorious research and simultaneously strengthen the research environment within our school. Overall, we hypothesize that selective disruption of the SERT in one or more of the proposed locations will recapitulate the skeletal phenotype observed with ubiquitously genetic disruption of SERT. Once the location/s for the skeletal effects of SERT inhibition is established, the stage will be set for future work exploring underlying mechanisms and pathways.